When Rensch Meets Bergmann: Does Sexual Size Dimorphism Change Systematically With Latitude?

Abstract

Abstract. Bergmann's and Rensch's rules describe common large-scale patterns of body size variation, but their underlying causes remain elusive. Bergmann's rule states that organisms are larger at higher latitudes (or in colder climates). Rensch's rule states that male body size varies (or evolutionarily diverges) more than female body size among species, resulting in slopes greater than one when male size is regressed on female size. We use published studies of sex-specific latitudinal body size clines in vertebrates and invertebrates to investigate patterns equivalent to Rensch's rule among populations within species and to evaluate their possible relation to Bergmann's rule. Consistent with previous studies, we found a continuum of Bergmann (larger at higher latitudes: 58 species) and converse Bergmann body size clines (larger at lower latitudes: 40 species). Ignoring latitude, male size was more variable than female size in only 55 of 98 species, suggesting that intraspecific variation in sexual size dimorphism does not generally conform to Rensch's rule. In contrast, in a significant majority of species (66 of 98) male latitudinal body size clines were steeper than those of females. This pattern is consistent with a latitudinal version of Rensch's rule, and suggests that some factor that varies systematically with latitude is responsible for producing Rensch's rule among populations within species. Identifying the underlying mechanisms will require studies quantifying latitudinal variation in sexspecific natural and sexual selection on body size.

Equal temperature–size responses of the sexes are widespread within arthropod species

“…Blanckenhorn et al [17]). Here, we also used two methods that examine change in relative body size within species, and that account for differences in size between species.…”

“…Rensch's rule (RR) states that male body size varies more than female body size, irrespective of which sex is larger. RR was originally formulated to describe the pattern seen across species within a related clade, but has since been tested within species to see if similar drivers exist at the intra-specific level [13,17]. Within a species, it predicts an increase in SSD with increasing body size in species where males are the larger sex, and a decrease in SSD with body size in species where females are larger [14,16].…”

A phylogenetic approach to the study of sexual size dimorphism in Felidae and an assessment of Rensch´s rule.

“…Consideramos que estudios intraespecíficos detallados del SSD son necesarios para develar las causas próximas y últimas que determinan dicho dimorfismo sexual en félidos, especialmente en especies de muy amplia distribución geográfica, ya que varias especies siguen la regla de Bergmann (Ashton et al 2000;Meiri et al 2004) y la variación geográfica del tamaño corporal puede constituirse en un factor importante en la determinación y variación del SSD en la naturaleza (Blanckenhorn et al 2006); estudios preliminares nuestros en varias especies de Felidae sugieren que esta situación puede ocurrir y merecen nuestra atención.…”

“…Si bien es importante en una primera aproximación, el verificar la existencia de SSD en un grupo y describir su dirección y magnitud, lo esencial es detectar aquellos factores que pueden estar determinándolo y afectándolo (características ecológicas, geográficas y de historia vital) así como establecer de que forma el grado de SSD puede a su vez influir sobre características de los organismos. (Moors 1980;Leutenegger y Cheverud 1982;Weckerly 1998;Isaac 2005;Blanckenhorn et al 2006).…”

The Evolution of Sexual Dimorphism: Understanding Mechanisms of Sexual Shape Differences

“…This is due to directional selection for a large body size and individuals with sub-optimal body sizes will have lower fitness [40,41]. Alternatively, there may be condition-dependence, where the larger sex is under stronger directional selection for a large size and will be more affected by different environmental factors as compared to the smaller sex.…”